The present invention generally relates to vehicle brakes and more particularly relates to an electrically operable brake system for automotive vehicles and a method for its actuation.
Automotive vehicles are typically equipped with a service brake system that is operable by a brake pedal and a parking brake system that is operable by a hand lever or a foot pedal. While the parking brake system is normally used to prevent a waiting or parked vehicle from rolling away, the objective of the service brake system is to reduce the speed of a vehicle during normal operation or to bring the vehicle to halt.
Modern service brake systems are frequently equipped with additional systems such as ABS, TCS, driving dynamics control systems, brake assist systems, and similar systems to improve the reliability in operation. Most recently, electrically operable brake systems for automotive vehicles have become known where the application of a brake pedal is sensed by a sensor device and converted into corresponding input signals for a control unit for the actuation of the service brake system. One disadvantage a control concept of such a type suffers from is, however, that upon failure of the sensor device which senses the brake pedal application, the service brake system fails as well and braking can be achieved only by way of the parking brake system which usually acts upon the rear axle. However, especially in braking a vehicle at higher speed which is effected by the parking brake system alone, the corresponding wheel brakes of the parking brake system are exposed to major load what can lead to considerable wear of the wheel brakes. Because the additional systems such as ABS, brake assist system, etc., do not act on conventional parking brake systems, the rear wheels may even lock, whereby driving stability can be impaired and the vehicle may go into a spin.
An object of the present invention is to provide an electrically operable brake system for automotive vehicles and a method for its actuation permitting an increased reliability and an improved operational safety.
In a particularly simple embodiment of the present invention, the additional brake operating device may be achieved e.g. by means of a manually operable switch which is mounted in the instrument panel, on the steering wheel, or any other appropriate location and is actuated by the driver in case of need. By actuating the switch which is normally envisaged for a case of emergency a corresponding brake signal can be sent to a control unit of the service brake system which, subsequently, presets an invariably adjusted nominal brake value, for example, in the way of deceleration, brake torque, tensioning force, etc. In this case, the hazard warning signal system could also be activated because this switch is mainly intended for emergencies. The brake operation can be discontinued by new actuation of the switch.
However, the control unit of the service brake system may also be designed so that when the switch is actuated, instead of a fixed nominal brake value or a fixed brake deceleration, a nominal brake value or a brake deceleration which increases continuously with the duration of the actuation is adjusted until a defined maximum value is reached. An increase of this type may be linear or progressive, for example.
It is also possible though that the deceleration or speed reduction is adjusted in response to the vehicle speed when the switch is actuated. The higher the vehicle speed, the more intense the braking intervention of the service brake system becomes.
In a particularly expedient embodiment of the present invention, the second brake operating device may also be a sensor unit with a plurality of sensors which have a preferably redundant and diverse configuration, by which e.g. the adjustment travel and the actuating force of a second brake operating element is sensed, which is configured as a lever, pedal, push button, or a like element. This permits predefining a brake signal to the control unit that is variable by the driver, which brake signal is processed in the control unit and converted into corresponding nominal brake values for the application of the wheel brakes. A controlled actuation of the service brake system may thus be achieved by way of the second brake operating element as well.
In another favorable embodiment, the second brake operating element may be adapted to the ergonomics of the hand and may comprise an active element which permits simulation of a feedback of the brake operation in the sense of a rising resistance force when the brake value increases. This imparts a feeling for the brake operation to the driver, which contributes to enhancing the operating comfort.
Under safety aspects, the mode of operation of the two brake operating devices is examined by a monitoring device which preferably has a redundant design, and an acoustic and/or visual alarm device can be activated, for example, upon detection of malfunction of a first brake operating device which is designed as a brake pedal simulator. This permits indicating to the driver that the foot operation of the service brake no longer functions and that the service brake must be activated by the e.g. manually operable second brake operating device.
In another especially appropriate embodiment of the present invention, it is also possible to use brake signals which are predetermined by a parking brake operating device for the actuation of a service brake system. In this arrangement, for example, the sensors and/or switches for sensing the operation of an electrically operable parking brake can be used, with the result of reduced costs. However, this case necessitates corresponding provisions for an allocation of the actuation of a parking brake operating element to the activation of the service brake system and/or parking brake system. A suitable allocation may be realized e.g. by way of the vehicle speed in such a manner that the service brake system is activated above a predetermined vehicle speed and the parking brake system is activated below the predetermined vehicle speed by actuation of the parking brake operating element. The result is that braking with the aid of the service brake system can be initiated by way of actuating the parking brake operating device even in case of failure of a service brake operating device which is e.g. configured as a brake pedal module. Intervention of the additional functions of the service brake system such as ABS, ESP, driving dynamics control, and like systems, may also take place. Further, it is ensured that the parking brake system will not be employed at higher vehicle speeds and stressed excessively thereby. It is nevertheless ensured that the driver can park his/her vehicle safely in standstill by means of the parking brake. Further special features and advantages of the present invention can be taken from the following description of advantageous embodiments by way of the accompanying drawings.